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NFκB

NFκB: Introduction

NFκB is a transcription factor, and a member of the Rel
family of proteins. In humans the genes encoding for NFκB are found at
locations 4q.23-34 and 11q.12-13, for p50 and 65 subunits respectively (OMIM).
The protein is normally found in the cytosol, bound non-covalently to an inhibitor
protein, IκB (Janeway et al. 2005). Originally discovered in B
cells as the transcription factor for the κ chain in immunoglobulins, NFκB
is known today to be involved in a wide variety of cell pathways including,
but not exclusively, stress induced, immune, and inflamatory responses (NFkb
component). All in all over 100 genes' transcriptions are promoted by NFκB
(Hiscott et al. 2001). The protein is also known to be involved in
the development of several cell types as well as having an active role in several
diseases.

NFκB: Activation

NFκB is commonly activated through a Toll pathway via TLR-4;
however several variations and non-classical pathways do exist that activate
NFκB. The cascade starts with a microbial substance (for this cascade assume
a lipopolysaccharide from Gram-negative bacteria) binding to CD14 which then
associates with TLR-4 on the surface of the cell. This association triggers
the cytoplasmic domain of TLR-4 (TIR) to bind and activate adaptor protein (MyD88),
which interacts with the death domain of serine/theronine innate immunity kinase
(SIIK), activating SIIK. SIIK then activates TRAF6 which in turn activates IκBα
and IκBβ to form a dimmer (Iκk) that phosphorylates IκB.
When phosphorylated IκB then dissociates from NFκB, allowing NFκB
to enter the nucleus and promote translation of genes (Janeway et al.
2005). These genes include (but are not exclusive to) IL-2, IL-2 receptor (IL-2R),
IL-8, interferons, Cyclin-D1 (a protein involved in the regulation og the G1
phase of the cell cycle), and many others (Imaginex).

Figure.1 This image provides the general signal activation
pathway for NFκB as described in the paragraph above. (McFadden 2006)

NFκB: Structure

NFκB has five subunits[p50 (TNFRSF5)/p105 (NFκB1),
p52/p100 (NFκB2), REL, p65/RELA and RELB]; however the p50/p65 heterodimer
is the most important for most responses (Pajonk et al. 2001). p50
and p65 dimerize around a 10 base pair region called the κB site. The binding
sequence of the κB site is 5'-GGGRNYYYCC-3' (R=purine, Y=pyrimidine, and
N=and base) (NFkb component).

Figure 2. This is a JMOL image of NFκB bound to a κB site
on a DNA molecule. Taken from the RSC PDB protein databank. (RSC
PDB 1VKX)

NFκB: Clinical Studies and Information

Many studies surrounding NFκB are centered on cancer. Studies
suggest that NFκB is a key component in providing the survival signal in
several types of cancer cells (Pajonk et al. 2001) . Activation of
NFκB promotes the production of several proteins involved in the survival
signal, countering the effects of TNF (Tumor Necrosis Factor) and other death
promoting proteins. NFκB has also been linked with potentially lethal damage
repair (PLD repair) in cells recieving radiation therapy, allowing the cells
to resist radiation-induced cell death. Inhibition of NFκB is a potentially
useful tool for increasing radiosensitivity of cancerous cells (Ding et
al. 2003). Normally high doses of radiation can activate NFκB, using
a non-classical pathway, providing cancer cells with a stronger survival signal
and more resistance to apoptosis. Inhibition of NFκB would make cells more
suceptable to signals transduced by TNF, Bax, Bad and other death promoting
proteins, because the survival signal that counters these signals would be weakened.
Also, lower rates of PLD repair are observed in cells where NFκB is inhibited.
Thus inhibition of NFκB would allow the DNA damage done by radiation to
stimulate apoptisis at a greater rate (Pajonk et al. 2001).

NFκB has implications in the activation of iNOS genes. iNOS
produces nitric oxide (NO), a powerful reducing agent. Reactive free radicals
like NO can cause serious damage to DNA by oxidizing the DNA molecule. This
type of DNA damage has been shown to lead to Alzheimer’s disease. Extensive
genetic data segust that β-amyloid peptide (Aβ) plays a role in the
neurodegenerative cascade of Alzheimer's disease. Aβ has also been noted
to activate NFκB. In Akama's study he claims that NO production occurs through
a NFκB dependent mechanism. While no mechanism for this cascade is known,
Akama supports his claim by showing that iNOS promoter activity is severly depressed
in cells with inactive NFκB (Akama et al. 1998).

NFκB has also been shown to play an essential role in HIV.
The efficient gene replication of HIV-1 is partially attributable to the ability
of the virus to sucessfully activate and bind NFκB to its gene sequence
(Hiscott et al. 2001). There are a few hypothesis to how HIV activates
NFκB. One study seguests that cleavage of Bcl-2 by the viral proteases leads
to activation of NFκB due to the inability of cleaved Bcl-2 to maintian
its normal level of supression of oxidative agents, and the oxidative sensitivity
of NFκB (Strack et al. 1996). It has also been proposed that a
cascade triggered by viral and cell membrane fusion involving CD4 activates
NFκB. Once active the NFκB complex binds to HIV LTRs (long terminal
repeats) at promoter regions, activating viral protease transcription (Hiscott
et al. 2001).

NFκB: Direct Drug Interactions

There are several drugs that can affect the activity of NFκB
through interactions with proteins that are involved in the cascades that regulate
NFκB; however those that interact with NFκB directly do so in two
common ways: directly blocking the activation of the NFκB complex, or directly
blocking the duffusion of the complex. Two such drugs are 6-Amino-4-(4-phenoxyphenylethylamino)quinazoline
(an inhibitor of the activation of NFκB), and NFκB SN50 (an inhibitor
of the diffusion of the NFκB complex into the nucleus). 6-Amino-4-(4-phenoxyphenylethylamino)quinazoline
was discovered by Tobe et. al. in 2003. Since then no literature has
been published reguarding the mecanism that is used by the drug to inhibit NFκB
activation (Liu 2006). NFκB SN50 inhibits diffusion of NFκB through
the neuclear membrane by binding to the complex and physically inhibiting its
diffusion (Puolaki et al. 2002).

Refferences

Akama KT, Albanese C, Pestell RG, Van Eldik LJ. Amyloid ß-peptide Stimulates
Nitric Oxide Production in Astrocytes through an NFkB-dependent Mechanism. Proceedings
of the National Academy of Sciences of the United States of America May 12,
1998 Vol. 95, No. 10, 5795-5800.